1. Field of the Invention
This invention relates generally to transducers which may be fabricated from semi-conductor material such as silicon by batch photolithographic techniques from a monocrystalline wafer. More particularly, the invention relates to a spring-mass-support structure for detecting a signal proportional to acceleration applied to the support structure. The invention also relates to electronic circuitry in combination with the structure for detecting displacement of the spring supported mass to produce analog or binary representations of acceleration applied to the support structure.
2. Description of the Prior Art
A publication by Kurt E. Petersen entitled "Silicon as a Mechanical Material" published in Vol. 70, No. 5 (May, 1982) of the Proceedings of the IEEE, surveys the art of micro-machining of silicon to fabricate a mechanical device. The Petersen publication describes etching techniques commonly used in fabricating micromechanical devices and describes several devices which have been made using such techniques. The publication also describes a cantilever beam light modulator array the beams of which are fabricated of silicon dioxide. Another survey article appeared in the April, 1983 publication of Scientific American entitled "Silicon Micromechanical Devices" by Angell, et al.
An accelerometer having a micromachined cantilever beam intended for in vivo biomedial studies and which includes a piezoresistive sensing element is described in a publication entitled "A Batch-Fabricated Silicon Accelerometer" by Roylance and Angell in IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, December, 1979.
U.S. Pat. No. Re. 31,459 (reissue of U.S. Pat. No. 4,071,838) reissued Dec. 6, 1983 in the name of Barry Block discloses a silicon folded leaf spring force transducer fabricated by batch photolithographic and etching techniques. One embodiment of such transducer consists of four E-springs located at 90.degree. angles about the central axis of sensitivity. Piezoresistors disposed on legs of the E-springs provide a measure of the displacement of the springs in response to a force applied to the transducer.
U.S. Pat. No. 4,155,516 issued Mar. 13, 1979, in the name of Harry Aine discloses first and second leaf spring structures, as supported from support structures, which are coupled together in axially spaced mutually opposed relation so that the first and second leaf spring structures are coupled together for deflection in unison relative to the surrounding support structures in response to deflection of either of the first and second spring structures. A capacitive detector structure is provided for detecting displacement of the leaf spring.
U.S. Pat. No. 4,597,003 issued June 24, 1986, in the names of Aine and Block discloses a method of chemical etching a silicon wafer by undercutting an etch stopped layer to fabricate a cantilever beam supported at one end by a frame structure with an integral mass formed by the etch. Etching from opposite etch stopped layers of a silicon wafer is described.
While the art described above has advanced the art of micro-machining of silicon wafers to fabricate sensors in general and accelerometers in particular, there remains a need for a micro-machined structure of silicon and other materials and associated electronics meeting extremely precise specifications. For example, extremely low distortion for an accelerometer is required for certain applications. Prior art spring configurations and materials are believed to produce spring constants which change unacceptably over a deflection range of interest. Very low sensitivity to accelerations orthogonal to the sensitivity axis is desired. Prior art spring mass configurations do not provide sufficient insensitivity to orthogonal acceleration and angular motion applied to such configurations. Very high dynamic range (requiring extremely low noise) is desired in an accelerometer which has not been provided by prior art accelerometers and associated detecting electronics systems. Shock impact insensitivity is required in an accelerometer intended for use in certain applications such as in the seismic, automotive, or aeronautics fields.